Technically it's 24999 exp. That said, 3 platinum armours and 9 black spirits can grant 24000 exp, so provided YoxalLoyal doesn't mind grinding her level a little before using them, they should be enough (not to mention it would be a waste to use four at that level). Some handy experience tables can be found here. Since it isn't made entirely clear in the thread, the "left" columns refer to how much experience is needed to reach the next level when a unit of the appropriate rarity is at the specified level (you want that experience remaining to be that number or lower).